Superconductivity is a well known phenomenon. For some time superconductivity has been considered as possibly beneficial in the various elements of microwave systems by permitting a substantial reduction of the ohmic resistance of such elements. With the discovery of availability of high temperature superconductive materials, workers in the field have assembled and tested various microwave devices to determine the extent to which superconductive elements may improve the performance of various microwave devices.
For example, Robert J. Dinger and David J. White of the Naval Weapons Center, in Theoretical Increase in Radiation Efficiency of Small Dipole Antenna Made with a High Temperature Superconductor, have reported a theoretical investigation of an electrically small dipole antenna with a shunted stub impedance matching network to determine the improvement in radiation efficiency that can be achieved by making the metallic components of high temperature superconducting materials. Their study was based on the discovery of Y-Ba-Cu-O ceramic materials that are superconducting at high enough temperatures to permit efficient cooling of relatively large antenna structures. The study considered dipoles with lengths up to 0.4 wavelengths and determined antenna input impedance for such dipoles as a function of radiation resistance, antenna element conductor loss and antenna reactance. Their study indicated that efficiency improved, as expected, as the surface resistance of the antenna elements was reduced, but that antenna efficiency leveled off when dielectric losses began to dominate, and that unless a dielectric loss tangent of less than 10.sup.-4 can be obtained, only a modest improvement of radiation efficiency can be obtained, and, in addition, even a dielectric loss tangent as low as 10.sup.-4 limits the high efficiency region to antenna elements with lengths of 0.2 wavelengths or higher. Messrs. Dinger and White concluded that the antenna and all supporting and matching structures must use very low loss dielectric materials to realize enhanced efficiency through superconductivity, that large standing waves will produce most of the system loss if low loss materials are not used, and that antenna ohmic losses may actually be only the smallest fraction of system losses. Dinger and White, therefore, suggested that dielectric materials should be avoided by using air dielectric lines and self-supporting antenna structures, but recognized that the high temperature superconductive materials have low thermal conductivities and will require heat transfer media of some type, and that these conflicting requirements complicated the design of practical superconductive, electrically-small antennas.
Raymond W. Conrad, in U.S. Statutory Invention Registration H653 published Jul. 4, 1989, disclosed a superconducting, superdirective array of half-wave dipoles constructed of various superconductive materials, including high temperature superconductive materials. Conrad's array comprised a plurality of half wave dipoles stacked with spacings of less than one-half wavelength of the emission frequency of the dipoles. The array was housed in a vacuum insulated container for the antenna array and the coolant, which is closed at one end by a microwave window for electromagnetic radiation. Conrad indicated the antenna elements must be made of a material with a high critical current and high critical magnetic field for maximum efficiency and further indicated that exceeding the critical current of the antenna element material will produce a return to normal conductivity and high ohmic losses which can damage the antenna element material.
Personnel of AT&T Laboratories have reported experiments with a 31 centimeter long, high temperature, superconducting, thin film microstrip transmission line comprised of a Y-Ba-Cu-O ground plane and microstrip line, both about 4000A thick, on lanthanate gallate substrates, separated by a sapphire dielectric. The microstrip pattern was a coiled serpentine arrangement with a line 125 micrometers wide and spaced 375 micrometers from adjacent coils. The sapphire dielectric was 125 micrometers thick providing a nominal line impedance of 50 ohms. Testing of the dc resistance of the line indicated a high quality film with a sharp transition to superconductivity at about 85.degree. K. The test results indicated little variation in signal delay and reflected wave shape at temperatures less than the critical temperature and that there are no basic changes in the basic characteristics of the transmission line at temperatures well below the critical temperature and currents below the critical current. See Experiments with a 31-CM High-Tc Superconducting Thin Film Transmission Line.
Lincoln Laboratory personnel used the discovery of high-temperature, superconducting materials to study stripline resonators and their use to stabilize oscillators operating in the 1 to 10 GHz range. The detection of small targets in clutter by doppler radar systems is currently limited by phase noise in the local oscillator, and a 10-20 db reduction in noise would provide a corresponding increase in sensitivity. Lincoln Laboratory personnel selected stripline techniques over microstrip techniques to eliminate radiation, permit planar fabrication techniques and provide a compact and rugged structure, and used the stripline resonator to study the surface temperature effects of the high temperature superconductive materials. As a result of their study, the Lincoln Laboratory personnel concluded that projected noise performance of stripline resonators with high temperature superconductors was better than competing technologies but was limited by flicker noise which may be due to the quality of the superconductive film.
Reissue U.S. Pat. No. 29,911 discloses a microstrip antenna structure that is formed from a unitary conducting surface separated from a ground plane by a dielectric substrate and providing radiating elements and feed lines. The disclosed antenna structure includes the provision of phased antenna arrays with high gains in the microwave region and phase shifting circuits obtained by using printed circuit techniques on a planar substrate.
Reissue U.S. Pat. No. 32,369 discloses an antenna system formed on a gallium arsenide semiconducting substrate with, where appropriate, feed networks, phasing networks, active or inactive semiconductor devices and microprocessor controllers. The gallium arsenide-base antenna system provides direct radiation and receiving elements and the phase-shifting, amplifying and controlling elements that can provide high gain phased arrays. In these systems microstrip radiators are provided by metallization adjacent a semiconductive material.
These reports and disclosures exemplify the extensive efforts to improve microwave system and antenna arrays.